Membrane systems are used for separation of gases, liquids from a mixture of gas and liquid streams. Multi-stage membranes are used to increase the product recovery or the product purity for commercial application. Generally, in a two-stage membrane system, the primary or pre-membrane and secondary membranes operate at different pressures. For example, in a conventional two-stage membrane system to recover the useful product, a permeate that passes through the primary membrane will be compressed and then will pass through the secondary membrane. The residue from the secondary membrane is then recycled to the primary membrane. This two-stage membrane configuration can significantly increase the product recovery compared to a one-stage membrane system. The two-stage system can also be used to obtain high purity permeate product. However, the required compressor power is generally high since the permeate from the primary membrane is compressed in this process configuration.
In some two-stage membrane systems, the permeate from the pre-membrane will not be compressed and sent to the secondary membrane, but instead, it will be directly sent to either the waste stream or product stream. Since the selectivity of the pre-membrane unit is very high, the membrane will provide either low product (as retentate) loss, or high purity product (as permeate). Other known methods include using an external energy resource to drive the compression and increase the pressure of the lower permeate stream to enable separation of the gas or liquid mixture streams. It is important to reduce the compression cost and costs involved in using external energy to drive the process of separation while maintaining the level of product recovery in any new configuration. It is further desirable to increase the retentate product recovery and/or permeate product purity without increase compression costs.